1. Field of the Invention
The present invention relates to fuel vapor processing apparatus for processing fuel vapor that may be produced within a fuel tank.
2. Description of the Related Art
A known fuel vapor processing apparatus is disclosed, for example, in Japanese Laid-Open Patent Publication No. 6-74107. The fuel vapor processing apparatus of this known publication is shown in FIG. 5 and includes a main canister 101 and a sub canister 103 connected in series with each other via a throttle 102. The main canister 101 communicates with an upper chamber of a fuel tank T via a fuel vapor passage 105 on one hand, and communicates with an intake air passage 108 of an engine E via a purge passage 106 and a control valve 107 on the other hand. The sub canister 103 is open into the atmosphere via an atmospheric port 103p. 
When the engine E is not started and is stopped, the control valve 107 is closed, and fuel vapor produced within the fuel tank T is introduced into the main canister 101 via the fuel vapor passage 105 so as to be adsorbed by an adsorption material K contained within the main canister 101. A part of the fuel vapor that has not been adsorbed within the main canister 101 is introduced into the sub canister 103 via the throttle 102 so as to be adsorbed by an adsorption material K contained within the sub canister 103. Therefore, the fuel vapor is prevented from dissipating into the atmosphere.
On the other hand, during the operation of the engine E, the control valve 107 is opened, so that a negative pressure within the intake air passage 108 is applied to the sub canister 103 via the main canister 101. Therefore, external air may enter the sub canister 103 via the atmospheric port 103p, so that the fuel vapor adsorbed by the adsorption material K within the sub canister 103 can be purged. In addition, the air that may flow into the main canister 101 via the throttle 102 can purge the fuel vapor adsorbed by the adsorption material K within the main canister 101. With the air flowing into the main canister 101, the purged fuel vapor is introduced into the intake air passage 108 via the purge passage 106.
According to the known fuel vapor processing apparatus, the fuel vapor adsorbed within the sub canister 103 is purged by the air that enters the sub canister 103 via the atmospheric port 103p and subsequently flows through the main canister 101 and the control valve 107. Therefore, if the control valve 107 is operated in a closing direction, for example, due to the control of the air-fuel ratio, during the operation of the engine E, the flow rate of the air entering the sub canister 103 via the atmospheric port 103p may be decreased. Hence, there is a possibility that the purging of the fuel vapor from within the sub canister 103 may not be effectively performed.
If the amount of the fuel vapor remaining within the sub canister 103 increases, there is a possibility that the fuel vapor is dissipated to the atmosphere via the atmospheric port 103p of the sub canister 103 when the engine E has stopped.
Therefore, there is a need in the art for fuel vapor processing apparatus that can prevent or minimize dissipation of fuel vapor into the atmosphere.